In Iraq, the United States faced down al-Qaeda and eventually dealt it a decisive defeat. In Syria, the al-Qaeda affiliate Jabhat al-Nusra is steadily gaining strength — and prompting, across the border, a revival of al-Qaeda in Iraq. The Obama administration’s hands-off approach offers no means for checking this menace or for preventing al-Qaeda from eventually gaining control over chemical and biological weapons.

The Iraq war prompted low-level meddling by Iran, Syria and other neighbors but otherwise left the surrounding region unscathed, thanks to the U.S. presence. Syria’s unchecked carnage is spilling over into Lebanon and Iraq, and it threatens U.S. allies Israel, Turkey and Jordan. Iran, Persian Gulf states and other neighbors are pouring in weapons and, in some cases, fighting units.

Iraq prompted a temporary souring of relations between the United States and France and Germany, and Arab Sunni monarchies never fully accepted the Shiite-led government that democracy produced. But U.S. influence in the Middle East remained strong. Now it is plummeting: Not just Britain and France but every neighbor of Syria has been shocked and awed by the failure of U.S. leadership. If it continues, Syria — not Iraq — will prove to be the turning point when America ceases to be regarded as what Bill Clinton called the “indispensable nation.”